The present invention relates generally to insulated, high voltage electrical conductors of the type provided with an intermediate layer of semi-conducting material otherwise known as a strand shield.
In the manufacture of insulated electrical conductors, a strand shield, i.e. a layer of semi-conductive material, is applied over a bare conductor before an outside jacket of insulating material is applied over the strand shield. The purpose of a shield is to minimize the creation of voltage gradients about and along the conductor caused by surface imperfections such as protrusions on the conductor surface. This can result in corona and deterioration of the dielectric properties of the insulation. The strand shield functions to negate such results by providing an equal potential field along and about the conductor.
A strand shield may be applied to the bare conductor by way of a head device capable of concentrically extruding semi-conductive material in a plastic state on the bare conductor, as the bare conductor moves concentrically through the head device. The strand shield and conductor then receive a jacket of insulating material, which is applied by a second extruding head located around or downstream from the head that applies the strand shield. Defects in the strand shield occur mostly in the second extruding head, and are caused by non-uniform pressure on the shield as it passes through an extruding tip located within the head, i.e. it is difficult to maintain a uniform concentric position and consequent flow of jacket material about the shield and conductor as they pass through the head applying the jacket material. The result of this is that the conductor and strand shield tend to move off center within the tip such that pressure is concentrated within the tip on the shield, and the shield tends to engage the tip, which damages and/or removes the strand shield from the conductor.
A device for detecting defects in strand shields is disclosed in U.S. Pat. No. 3,841,810 to Robinson et al, this patent disclosing the use of a direct current (DC) voltage applied between the conductor and the tip of the extruding head. The tip is electrically insulated from the remainder of the head and the extruding apparatus and machinery. Current flow in the conductor that might be caused by a break or other defect in the strand shield is measured by a resistance bridge circuit connected across the DC source and between the conductor and the insulated tip.